The technical part of the optician's profession consists in fitting a pair of corrective ophthalmic lenses in a spectacle frame selected by a wearer.
This fitting breaks down into three main operations:                acquisition of the contours of the surrounds of the selected spectacle frame,        centering of the lenses which consists in transferring the two acquired contours onto the two ophthalmic lenses, in a position and according to an orientation such that, once fitted in the spectacle frame, the lenses are appropriately centered facing the corresponding pupils of the wearer, then        machining of the lenses, which consists in cutting them along these contours.        
In the context of the present invention, interest is more particularly focused on half-rim spectacle frames, which comprise two half-rims (or “arches”), each closed by a thread usually made of nylon.
These spectacle frames are generally delivered to the optician with two presentation lenses.
For the acquisition operation, the optician uses a contour reading appliance which comprises a rod-shaped feeler, arranged so that its cylindrical face can slide along the edge of each presentation lens. This appliance is used to determine the planar coordinates, that is to say the coordinates in two dimensions, of a longitudinal profile running along each presentation lens.
As for the machining operation, this consists in cutting the ophthalmic lens along a trajectory derived from said planar coordinates, so that this lens can be mechanically and esthetically adapted to the shape of the surround of the selected frame, while best performing the optical function for which it was designed.
The machining operation comprises in particular a groove-forming step, for forming a fitting groove on the edge of the lens. When fitting the lens in the surround, this fitting groove is engaged on a rib that runs along the internal face of the corresponding arch of the frame. The ophthalmic lens is then kept pressed against this arch by means of the nylon thread which is engaged in the fitting groove.
The acquisition and machining operations must be performed with care so that each lens can be perfectly fitted into its surround, without force and “at the first attempt”, that is to say, without requiring remachining or bending the nylon thread.
Despite the care paid to these operations, it is observed that some ophthalmic lenses remain difficult to fit in their surrounds. It is then necessary, to dispel any risk of the lens slipping out of its surround, to remachine the lens and/or modify the length of the nylon thread, which is tedious to do.